1. Field of the Invention
This invention relates to the preparation of poly-isocyanurate foams from polyisocyanates and oxypropylated polyols, and is more particularly concerned with the use of these polyisocyanurate foams in the production of insulating panels.
2. Description of the Prior Art
Cellular polyurethane polymers are well known in the art for their use in various types of thermal insulating applications, such as refrigeration insulation, pipe and tank insulation, structural uses, e.g., preformed rigid panels, and aircraft structural parts. In these applications, the rigid polyurethane foams display excellent properties, particularly low thermal conductivity, but a great demerit of these foams is their combustibility. Various methods imparting flame-resistance to the polyurethane foams have been developed, and to date flame retardant urethane foam systems have dominated various markets for insulating materials, such as for walk-in cooler panels, building panels and pipe insulation.
It has been proposed to replace the conventional urethane foams with rigid polyisocyanurate foams which have both urethane and isocyanurate groups and good thermal stability, as well as excellent insulating capacity. While rigid isocyanurate foams, in general, are superior to rigid polyurethane foams in terms of flame resistance, smoke evolution, and thermal degradation properties, various problems have been encountered with the polyisocyanurates, such as excessive brittleness, poor abrasion resistance and poor molding characteristics. Consequently, these foams have not found wide acceptance in replacing polyurethanes in the production of insulating panels for the refrigeration industry and the like.
Futhermore, isocyanurate foam systems are historically fast reacting with cream and firm tiems of 20-60 sec. Such fast reactivities can be a mojor obstacle in molding large foam panel sections, resulting in non-uniform products of low strength. Unfortunately, the use of catalysts for slowing these conventional systems down to appreciably longer firm times results in foams having unacceptable properties, e.g., friability readings greater than about 25% and poor flow characteristics.
It would accordingly be highly desirable to provide an improved process for preparing polyisocyanurate foams of good quality which are particularly suited for use in insulating panels.